Fruit and insects abound all year in the forests of New Guinea, the largest tropical island in the world, and natural threats are few. Linked to Australia until about 8,000 years ago, the 1,500-mile-long (2,400 kilometers) island shared much of its neighbor's fauna. Marsupials and birds were plentiful, but placental mammals were entirely absent, meaning no monkeys and squirrels to compete with birds for food, and no cats to prey on them. The result: an avian paradise that today is home to more than 700 species of birds.

Freed of other pressures, birds of paradise began to specialize for sexual competition. Traits that made one bird more attractive than another were passed on and enhanced over time. Known as sexual selection, this process "is to birds of paradise what natural selection is to Darwin's finches—the prime mover," says Scholes. "The usual rules of survival aren't as important here as the rules of successful mating."

The diversity of New Guinea's birdlife also springs from its wealth of habitats, from humid coastal savannas to high-elevation cloud forests. Tangled swamps checker the lowlands, while a spine of rugged mountains, some rising 16,000 feet (5,000 meters), creates a labyrinth of scarp and crag in the remote interior. Shaped by volcanoes, earthquakes, and equatorial rains, the landscape is rife with physical barriers that isolate wildlife populations, allowing them to diverge into new species. (The fractured landscape is also reflected in the diversity of indigenous cultures; more than 750 languages are spoken just in Papua New Guinea, the eastern half of the island.)

Much of New Guinea remains wild as ever, its fauna still not fully explored. In December 2005 scientists surveying the Foja Mountains in Indonesia's Papua Province, the western half of the island, came upon the Berlepsch's parotia, a bird of paradise with half a dozen springy feathers on its head. This legendary species was previously known only from a few partial specimens collected more than a century ago.

Farther east, in Papua New Guinea's Crater Mountain reserve, the forest grows dense to the mountain's summit, forming a canopy that blocks all but the thinnest rays of sun. Birdsong rings out in the gloom, a hoot here, a trill there, a melodious whistle, a harmonic tone as when a finger circles the rim of a glass. Drenched by nearly 300 inches (760 centimeters) of rain a year, this highland terrain is forever dripping. The forest floor, composed of layer on layer of organic material, is a wet sponge underfoot. And always, from somewhere below, comes the muted rush of a cold river spiriting away last night's rain.

Trails are rutted and mud-slick, swallowing boots and bruising the ankles of a first-time visitor. But the local women and children, who for a few kina will carry heavy gear and even lead you by the hand, tread lightly on bare feet. Pull out pictures of what you're looking for, and the men will lead you on long, clambering hikes, their machetes swinging to clear a path to where the birds of paradise hold court.